Various terms are defined in the following specification. For convenience, a Glossary of terms is provided herein, immediately preceding the claims.
Liquefied natural gas (LNG) is typically stored at cryogenic temperatures of about −162° C. (−260° F.) and at substantially atmospheric pressure. As used herein, the term “cryogenic temperature” includes any temperature of about −40° C. (−40° F.) and lower. Typically, LNG is stored in double walled tanks or containers. The inner tank provides the primary containment for LNG while the outer tank holds insulation in place and protects the inner tank and the insulation from adverse effects of the environment. Sometimes, the outer tank is also designed to provide a secondary containment of LNG in case the inner tank fails. Typical sizes of tanks at LNG import or export terminals range from about 80,000 to about 160,000 meters3 (0.5 to 1.0 million barrels) although tanks as large as 200,000 meters3 (1.2 million barrels) have been built or are under construction.
For large volume storage of LNG, two distinct types of tank construction are widely used. The first of these is a flat-bottomed, cylindrical, self-standing tank that typically uses a 9% nickel steel for the inner tank and carbon steel, 9% nickel steel, or reinforced/prestressed concrete for the outer tank. The second type is a membrane tank wherein a thin (e.g. 1.2 mm thick) metallic membrane is installed within a cylindrical concrete structure which, in turn, is built either below or above grade on land. A layer of insulation is typically interposed between the metallic membrane, e.g., of stainless steel or of a product with the tradename Invar, and the load bearing concrete cylindrical walls and flat floor.
While structurally efficient, circular cylindrical tanks in their state-of-practice designs are difficult and time consuming to build. Self-standing 9% nickel steel tanks, in their popular design where the outer secondary container is capable of holding both the liquid and the gas vapor, albeit at near atmospheric pressure, take as long as thirty six months to build. Typically, membrane tanks take just as long or longer to build. On many projects, this causes undesirable escalation of construction costs and length of construction schedule.
Recently, radical changes have been proposed in the construction of LNG terminals, especially import terminals. One such proposal involves the building of the terminal a short distance offshore where LNG will be off-loaded from a transport vessel, and stored for retrieval and regasification for sale or use as needed. One such proposed terminal has LNG storage tanks and regasification equipment installed on what is popularly known as a Gravity Base Structure (GBS), a substantially rectangular-shaped, barge-like structure similar to certain concrete structures now installed on the seafloor and being used as platforms for producing petroleum in the Gulf of Mexico.
Unfortunately, neither cylindrical tanks nor membrane tanks are considered as being particularly attractive for use in storing LNG on GBS terminals. Cylindrical tanks typically do not store enough LNG to economically justify the amount of room such tanks occupy on a GBS and are difficult and expensive to construct on a GBS. Further the size of such tanks must typically be limited (e.g. to no larger than about 50,000 meters3 (approximately 300,000 barrels)) so that the GBS structures can be fabricated economically with readily available fabrication facilities. This necessitates a multiplicity of storage units to satisfy particular storage requirements, which is typically not desirable from cost and other operational considerations.
A membrane-type tank system can be built inside a GBS to provide a relatively large storage volume. However, a membrane-type tank requires a sequential construction schedule wherein the outer concrete structure has to be completely built before the insulation and the membrane can be installed within a cavity within the outer structure. This normally requires a long construction period, which tends to add substantially to project costs.
Accordingly, a tank system is needed for both onshore conventional terminals and for offshore storage of LNG, which tank system alleviates the above-discussed disadvantages of self-standing cylindrical tanks and membrane-type tanks.
In published designs of rectangular tanks (see, e.g., Farrell et. al., U.S. Pat. Nos. 2,982,441 and 3,062,402, and Abe, et al., U.S. Pat. No. 5,375,547), the plates constituting the tank walls that contain the fluids are also the major source of strength and stability of the tank against all applied loads including static and, when used on land in a conventional LNG import or export terminal or a GBS terminal, earthquake induced dynamic loads. For such tanks, large plate thickness may be required even when the contained liquid volume is relatively small, e.g., 5,000 meters3 (30,000 barrels). For example, Farrell et al. U.S. Pat. No. 2,982,441 provides an example of a much smaller tank, i.e., 45,000 ft3 (1275 meters3), which has a wall thickness of about ½ inch (see column 5, lines 41–45). Tie rods may be provided to connect opposite walls of the tank for the purpose of reducing wall deflections and/or tie rods may be used to reinforce the corners at adjacent walls. Alternatively, bulkheads and diaphragms may be provided in the tank interior to provide additional strength. When tie rods and/or bulkheads are used, such tanks up to moderate sizes, e.g., 10,000 to 20,000 meters3 (60,000 to 120,000 barrels), may be useful in certain applications. For traditional use of rectangular tanks, the size limitation of these tanks is not a particularly severe restriction. For example, both Farrell, et al., and Abe, et al., tanks were invented for use in transport of liquefied gases by sea going vessels. Ships and other floating vessels used in transporting liquefied gases typically are limited to holding tanks of sizes up to about 20,000 meters3.
Large tanks in the range of 100,000 to 200,000 meters3 (approximately 600,000 to 1.2 million barrels), built in accordance with the teachings of Farrell et al. and Abe, et al. would require massive interior bulkheads and diaphragms and would be very costly to build. Typically, any tank of the type taught by Farrell et al., and Abe, et al., i.e., in which the tank strength and stability is provided by the liquid containing tank exterior walls or a combination of the tank interior diaphragms and liquid containing tank exterior walls, is going to be quite expensive, and most often too expensive to be deemed economically attractive. There are many sources of gas and other fluids in the world that might be economically developed and delivered to consumers if an economical storage tank were made available.
Bulkheads and diaphragms in the interior of a tank built in accordance with the teachings of Farrell, et al. and Abe, et al., would also subdivide the tank interior into multiple small cells. When used on ships or similar floating bodies, small liquid storage cells are of advantage because they do not permit development of large magnitudes of dynamic forces due to ocean wave induced dynamic motion of the ship. Dynamic motions and forces due to earthquakes in tanks built on land or on sea bottom are, however, different in nature and large tank structures that are not subdivided into a multitude of cells typically fare better when subjected to such motions and forces.
Accordingly, there is a need for a storage tank for LNG and other fluids that satisfies the primary functions of storing fluids and of providing strength and stability against loads caused by the fluids and by the environment, including earthquakes, while built of relatively thin metal plates and in a relatively short construction schedule. Such a tank will preferably be capable of storing 100,000 meters3 (approximately 600,000 barrels) and larger volumes of fluids and will be much more fabrication friendly than current tank designs.